The heart is comprised of different cell types, including the sino-atrial node (SAN) and contractile myocardium. The substantial function difference between these cell types are dictated by a unique subset of proteins and that identification of these groups of proteins will provide important information on how heart function is performed and regulated. Protein samples were extracted from isolated SAN cells, SAN tissue, right atria and left ventricle tissues and subjected to 2-D gel electrophoresis (pH 4-7 and 6-11; 10% bis-TRIS gel). More than 600 spots were analyzed, producing a non-redundant protein list of 250 proteins, with 20 identified proteins unique for a specific cell type. For example, a group of calcium-handling proteins, such as annexin A2, annexin A4, annexin A5, have higher abundance in SAN. They may play important roles in regulating heart beat. Neurofilament 3 is present only in SAN and not in atria and ventricles. On the other hand, collagen 1 has higher abundance in atria than ventricle myocardium but is not present in the SAN cells. Based on relative quantities, it was clear that there are differences between the cell types in the protein classes indicating phenotypic functional requirements.